Synthesis and use of ortho-(branched alkoxy)-tert-butoxybenzenes

Article abstract of DOI:10.1016/j.tetlet.2012.10.010

A series of sterically hindered o-(branched alkoxy)-tert-butoxybenzenes was efficiently prepared in good yields owing to a new practical and simple preparation of o-tert-butoxyphenol starting from catechol and isobutene. Use of DMF di-tert-butyl acetal reagent instead of isobutene/H₂SO ₄ (cat.) for O-tert-butylation was very convenient in case of ortho bulky phenols affording the corresponding tert-butyl ethers in high yield and purity. This general route proved to be useful since no reliable access was available to o-di-t-BuO-substituted arenes. Application to the synthesis of congested phosphorus-based compounds is presented.

Full text of DOI:10.1016/j.tetlet.2012.10.010

Tetrahedron Letters xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and use of ortho-(branched alkoxy)-tert-butoxybenzenes
⇑
⇑
Slavko Rast, Michel Stephan , Barbara Mohar
National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of sterically hindered o-(branched alkoxy)-tert-butoxybenzenes was efficiently prepared in good
yields owing to a new practical and simple preparation of o-tert-butoxyphenol starting from catechol and
isobutene. Use of DMF di-tert-butyl acetal reagent instead of isobutene/H2SO4 (cat.) for O-tert-butylation
was very convenient in case of ortho bulky phenols affording the corresponding tert-butyl ethers in high
yield and purity. This general route proved to be useful since no reliable access was available to o-di-t-
BuO-substituted arenes. Application to the synthesis of congested phosphorus-based compounds is
presented.
Received 29 August 2012
Revised 21 September 2012
Accepted 2 October 2012
Available online xxxx
Keywords:
Dialkoxyarene
tert-Butoxyarene
o-Bromo-tert-butoxyarene
DMF di-tert-butyl acetal
Phosphine
Ó 2012 Elsevier Ltd. All rights reserved.
Despite the availability of a variety of simple-to-perform syn-
our hands (Scheme 1, Route A). A closer look at their published data
revealed that the corresponding 1H NMR9 does not match with the
expected compound profile compared to a well-known series of o-
dialkoxybenzenes.7,10 Facing this discrepancy,11 we ventured to
synthesize the symmetric o-di-tert-butoxybenzene and its closely
related o-(branched alkoxy)-tert-butoxybenzenes which results
are presented herein.
We initiated our investigation by studying the reaction of cate-
chol with isobutene under milder conditions (rt to 35 °C) employ-
ing a catalytic amount of H2SO4 in an aprotic solvent (PE 100–120,
heptane, or CH2Cl2) (Scheme 1, Route B). Such conditions were
thetic routes toward tert-butoxyarenes,1–3 still access to ortho con-
gested ones represents
a
particular challenge in organic
chemistry.4 Among the devised strategies to the former class of
compounds, the acid-catalyzed addition of phenols to isobutene
is the most economical and practical.2 By contrast, a comprehen-
sive literature survey revealed only a very limited number of
poorly characterized o-di-t-BuO-substituted arenes; as for
example, the fully peralkoxylated hexa-tert-butoxybenzene and a
densely substituted 2,3-di-tert-butoxynaphthalene which were
prepared though via aromatic cyclization reactions.5
Pursuing our study of bulky P-based compounds,6 we aimed to
increase the vicinal steric crowding of the appended P-o-RO-Ph
groups wherein R represents a branched alkyl. Therefore, it was
of special interest to prepare a series of o-(branched alkoxy)-tert-
butoxybenzenes.7
To the best of our knowledge, the preparation of o-di-tert-but-
oxybenzene and its further use in phosphines’ synthesis has been
only reported by Horner and Simons in 1983.8 They have also de-
scribed therein the para isomer claiming their both preparations
via the H2SO4-catalyzed addition of the corresponding diphenol
to isobutene at 70 °C. However, our repetitive attempts to prepare
the requisite o-di-tert-butoxybenzene according to their published
procedure were consistently unsuccessful as a variety of ring tert-
butylated catechols (variable yields) was being formed instead in
⇑
Corresponding authors. Present address: PhosPhoenix SARL, 115, rue de l’Abbé
Groult, F-75015 Paris, France (M.S.). Tel.: +386 1 4760250; fax: +386 1 4760300.
E-mail addresses: mstephan@phosphoenix.com (M. Stephan), barbara.mohar@
ki.si (B. Mohar).
Scheme 1. O-tert-Butylation of catechol under various conditions.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.010
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2
S. Rast et al. / Tetrahedron Letters xxx (2012) xxx–xxx
already recommended by Stevens back in 1955 for the preparation
of mono tert-butoxyarenes.12
While the rt reaction in CH2Cl2 of PhOH with excess isobutene
in the presence of a trace of H2SO4 (performed in a glass pres-
sure-bottle) led in our hands to the major formation of 4-tert-bu-
tyl- and 2,4-di-tert-butylphenol mixture wherein Friedel–Crafts
C-alkylation had taken place,12 interestingly enough starting from
catechol (reaction in CH2Cl2 at 35 °C) and neutralizing H2SO4 with
Et3N after 3 h, furnished o-tert-butoxyphenol (1) in 92% (86% iso-
lated yield) accompanied by 8% of the targeted O,O0-di-tert-butyl-
ated product 2 (by 1H NMR analysis). In the literature, 1 has
been prepared in 23–33% overall yield via a protection–etherifica-
tion–deprotection sequence from catechol.13 We have noticed that
a reaction equilibrium exists during O-tert-butylation of 1 (protec-
tion of catechol remaining free OH group) as prolonging reaction
time did not lead to a significant yield increase of O,O0-di-tert-
butylcatechol (2), but to isobutene polymerization. Advanta-
geously, neutralizing H2SO4 prior to reaction workup proved to
prevent C-tert-butylation as well as de-O-tert-butylation. Our
study showed that 1 rearranges readily into isomeric 4-tert-butyl-
and 3-tert-butylcatechols (4-TBC/3-TBC 9:1 ratio) in heptane at rt
in the presence of a trace of H2SO4. Also, employing CH2Cl2 instead
of heptane or petroleum ethers was beneficial for reaction rate due
to a better catechol solubilization. Further on, instead of the acid-
catalyzed O-tert-butylation of phenols using isobutene, the non-
acidic working conditions (toluene at 80–110 °C) provided by the
commercially available DMF di-tert-butyl acetal reagent14
(ꢀ8 equiv) permitted finally the formation of o-di-tert-butoxyben-
zene (2) from 1 in 72% isolated yield. Thus, following the above
optimized protocols, o-tert-butoxyphenol (1) and o-di-tert-butoxy-
benzene (2) were efficiently prepared from catechol on multigram-
scale (up to 20 g).
Scheme 3. Oxidation of o-tert-butoxyphenyllithium.
In parallel, alternative strategies to o-di-tert-butoxybenzene (2)
from catechol were explored but to no avail. Applying Bandgar’s
conditions (Zn in t-BuCl, rt)2f or Bartoli’s conditions (5 mol%
Sc(OTf)3 in excess Boc2O, rt)2g,2h resulted in the exclusive forma-
tion of 3,5-di-tert-butylcatechol (3,5-DTBC) in quantitative isolated
yield within 1 day for the former and in ꢀ20% isolated yield (30%
total conversion) after 2 days for the latter (Scheme 2).
Another potential stepwise route to o-di-tert-butoxybenzene
(2) was attempted inspired from the preparation of tert-butoxy-
benzene via PhMgBr addition to PhCO2Ot-Bu.3a Surprisingly, the
ortho-metallated tert-butoxybenzene (metal = Li or MgBr) reacted
with PhCO2Ot-Bu to yield 2-tert-butoxy-benzophenone (4) in 52%
isolated yield with concomitant formation of o-tert-butoxyphenol
(1) and PhCO2t-Bu (Scheme 3). Nevertheless, o-tert-butoxyphenol
could be prepared in 65% yield via o-tert-butoxyphenyllithium
addition to B(OMe)3 followed by oxidation.
Next, we were interested to extend this study to other phenols.
Thus, the mild O-tert-butylation conditions employing isobutene/
H2SO4 (cat.) in CH2Cl2 at <40 °C (with low-temperature H2SO4
neutralization using Et3N) were applied to o-tert-butylphenol,
3,5-di-tert-butylcatechol, 3-bromocatechol, 4,5-dibromocatechol,
2,3-dihydroxynaphthalene, 4,6-di-tert-butylresorcinol, and pyro-
gallol (Scheme 4). However, a complex mixture was obtained in
the case of o-tert-butylphenol and pyrogallol.
Scheme 4. O-tert-Butylation of selected dihydroxyarenes using isobutene/H2SO4
(cat.) in CH2Cl2 at 35–40 °C and low-temperature acid neutralization with Et3N
(after 0.5–7 days).
Similarly as noted in the case of catechol, an equilibrium was
observed in the H2SO4-catalyzed addition of the examined series
to isobutene, especially during the second O-tert-butylation; pro-
longing reaction time did not favor a significant yield increase of
the O,O0-di-tert-butylated product. Fortunately, the latter could
be obtained in good yield reverting to the DMF di-tert-butyl acetal
reagent. Steric hindrance and electron-poor catechols had detri-
mental effect on O-tert-butylation using isobutene/H2SO4 in gen-
eral, and on O,O0-di-tert-butylation in particular. For example, 3 h,
3 days and 12 h were necessary for a reasonable conversion to
mostly mono O-tert-butylated products of catechol, 3-bromocate-
chol, and 4,5-dibromocatechol, respectively.
In particular, O-tert-butylation of 3-bromocatechol led to a mixture
of 2-bromo-6-tert-butoxyphenol (5), 3-bromo-2-tert-butoxyphenol
Scheme 2. Attempts to O-tert-butylate catechol under selected literature condi-
tions applied toward tert-butoxybenzene.
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S. Rast et al. / Tetrahedron Letters xxx (2012) xxx–xxx
3
(6), and 2,3-di-tert-butoxy-bromobenzene (7) in 72:22:6 ratio, accom-
panied with an unreacted starting material. Noteworthy, forcing
O-tert-butylation under the same conditions of 2-bromo-6-tert-but-
oxyphenol (5) (prepared via another route, vide infra) resulted in no
conversion to the desired diether. Hence, it can be concluded that
O,O0-di-tert-butylated product 7 is derived from 6.
2,3-Dihydroxynaphthalene yielded after 7 days a mixture of
3-tert-butoxy-2-hydroxynaphthalene (10), 2,3-di-tert-butoxy-
naphthalene (11), and 3-tert-butoxy-1-tert-butylnaphthalen-2-
one (12)15 in 70:15:15 ratio. As H2SO4-catalyzed rearrangement
of 10 gave 6-tert-butyl-2,3-dihydroxynaphthalene (13), the forma-
tion of 12 could stem either from a possible C-tert-butylation of a
ketonic form of 2,3-dihydroxynaphthalene or a direct aromatic C-
tert-butylation followed by enol-keto tautomerization to accom-
modate the bulky tert-butyl group.
Scheme 7. Preparation of congested P-based compounds calling upon ortho-(
branched alkoxy)-tert-butoxybenzenes.
a-
Pursuing the functionalization of o-tert-butoxyphenol (1), its
Williamson’s etherification in acetone using the reactive 3-bromo-
cyclohexene or i-PrI and K2CO3, furnished after 2–3 days the tar-
geted dissymmetric diethers in 85% and <5% yields, respectively
(Scheme 5). Using NaH in excess with i-PrI as the solvent and per-
forming the reaction in a glass pressure-tube at 75 °C for 2–3 days
afforded the o-tert-butoxy-iso-propoxybenzene (16) in up to 91%
yield. Further on, PtO2-catalyzed hydrogenation of o-tert-butoxy-
(2-cyclohexenyloxy)benzene (17) afforded o-tert-butoxy-cyclo-
hexyloxybenzene (18) in 78% isolated yield.
The high yield generation of ortho-lithiated 1,2-dialkoxyarene
species used in phosphines’ synthesis, can be conveniently facili-
tated under mild conditions via bromine–lithium exchange. More-
over, regioselective arene bromination is crucial when utilizing
unsymmetrical 1,2-dialkoxyarenes with two non-equivalent and
competing free ortho-positions. Thus, the contiguous bromine
atom-incorporated 2,3-di-tert-butoxybenzene 7 and analogue 20
(Scheme 6) were prepared in good yields by O-tert-butylation
using excess DMF di-tert-butyl acetal of the o-bromo-hydroxya-
renes derived in turn from ortho-regioselective ring bromination
(t-BuNH2/Br2)16 of o-(branched alkoxy)phenols. Unfortunately,
2-bromo-6-neopentoxyphenol (19) resisted O-tert-butylation un-
der these standard conditions. Noteworthy, O-tert-butylation with
DMF di-tert-butyl acetal of 1 or 5 proceeded with comparable reac-
tion rates.
Figure 1. ORTEP drawing of 22 at the 50% probability level.
obtained in 93–96% yields. In another synthetic variant for 22
(29% overall yield), the diphenylphosphino group was first intro-
duced starting from 5 followed by O-tert-butylation with DMF
di-tert-butyl acetal. 1H NMR of the P-(2,3-di-tert-butoxyphenyl)-
substituted phosphine 22 is in utter disagreement with the
corresponding one reported by Horner and Simons8 bringing an
additional proof against their misleading claim concerning prepa-
ration of both o-di-tert-butoxybenzene (2) and its P-derivative.
Our characterization of 22 is backed up by its X-ray crystal struc-
ture determination (Fig. 1).17 Progressing toward bulky P-stereo-
genic P-based compounds, we have prepared the stereomerically
pure P-(2,3-di-tert-butoxyphenyl)-appended aminophosphine-
P-borane 24 by ring-opening of enantiopure (–)-oxazaPB with
preformed 2,3-di-tert-butoxyphenyllithium.18,19 Such intermedi-
ate (24) serves as key-precursor in the Jugé–Stephan asymmetric
route to P-stereogenic phosphines.20 Preparation of derived bulky
phosphines and their application in asymmetric catalysis will be
presented elsewhere.
Finally, incorporation of the bulky 3-(
tert-butoxyphenyls onto P-based compounds was undertaken
(Scheme 7). Starting from the corresponding 3-( -branched alk-
a-branched alkoxy)-2-
a
oxy)-2-tert-butoxy-bromobenzenes 7 and 20, (3-RO-2-tert-butoxy-
phenyl)diphenylphosphines 22 (R = t-Bu) and 23 (R = i-Pr) were
Scheme 5. Williamson’s etherification of o-tert-butoxyphenol (1).
In summary, taking advantage of a new simple and straightfor-
ward preparation of o-tert-butoxyphenol, we have succeeded to
prepare in high yields the elusive o-di-tert-butoxybenzene along
with a series of congested o-RO-tert-butoxybenzenes wherein
R = i-Pr, 2-cyclohexenyl, and Cy. In addition, the DMF di-tert-butyl
acetal reagent proved to be quite effective for O-tert-butylation of a
variety of congested phenols. Thus, starting from catechols, isobu-
tene/H2SO4 (cat.) system can be applied for the preparation of
o-tert-butoxyphenols (major mono O-tert-butylation), while when
Scheme 6. Synthesis of 3-(branched alkoxy)-2-tert-butoxy-bromobenzenes.
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4
S. Rast et al. / Tetrahedron Letters xxx (2012) xxx–xxx
5. (a) Semmelhack, M. F.; Park, J. Organometallics 1986, 5, 2550–2552; (b)
Serratosa, F. Acc. Chem. Res. 1983, 16, 170–176; (c) Enhsen, A.; Karabelas, K.;
Heerding, J. M.; Moore, H. W. J. Org. Chem. 1990, 55, 1177–1185; (d) Causey, P.
W.; Dubovyk, I.; Leznoff, C. C. Can. J. Chem. 2006, 84, 1380–1387.
targeting o-di-tert-butoxyarenes, DMF di-tert-butyl acetal reagent
can be practical starting from o-tert-butoxyphenols. Moreover,
successful use of the bulky ortho-(a-branched alkoxy)-tert-butoxy-
6. (a) Stephan, M.; Mohar, B. FR2887253, 2005; WO2006136695, 2006 (http://
www.cas.org/spotlight/rlist1q07p/rlist1q07p.html); Chem. Abstr. 2006, 146,
82028; (b) Stephan, M.; Šterk, D.; Mohar, B. Adv. Synth. Catal. 2009, 351,
benzenes for the preparation of achiral and chiral P-based com-
pounds was accomplished.
ˇ ˇ
2779–2786; (c) Zupancic, B.; Mohar, B.; Stephan, M. Org. Lett. 2010, 12, 1296–
ˇ ˇ
1299; (d) Zupancic, B.; Mohar, B.; Stephan, M. Org. Lett. 2010, 12, 3022–3025;
Acknowledgments
ˇ ˇ
(e) Stephan, M.; Šterk, D.; Zupancic, B.; Mohar, B. Org. Biomol. Chem. 2011, 9,
5266–5271.
This work was supported by the Ministry of Higher Education,
Science, and Technology of the Republic of Slovenia (Grant No.
P1-242). We thank Barbara Modec, University of Ljubljana, for X-
ray crystal structure determination on the Supernova diffractome-
ter of EN-FIST Centre of Excellence, Slovenia.
7. For 1H NMR spectra of bulky o-di-RO-benzenes wherein R = i-Pr, i-Bu, Cy, etc,
ˇ ˇ
see: Stephan, M.; Zupancic, B.; Mohar, B. Tetrahedron 2011, 67, 6308–6315.
8. Horner, L.; Simons, G. Phosphorus, Sulfur Silicon Relat. Elem. 1983, 14, 189–209.
9. The reported8 1H NMR does not correspond to the one for o-di-tert-
butoxybenzene.
10. For 1H NMR spectra of o-di-RO-benzenes wherein R = Me and Et, see: Pouchert,
C. J.; Behnke, J. The Aldrich Library of 13C and 1H FT-NMR Spectra; Aldrich
Chemical Co.: Milwaukee, WI, 1993.
Supplementary data
11. For a similar discrepancy against Ref. 8 regarding their erroneous claim for p-
di-tert-butoxybenzene preparation, see: Carman, R. M.; Kanizaj, N.; Taylor, R. J.
K. Aust. J. Chem. 1997, 50, 515–516.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.10.
010. These data include MOL files and InChiKeys of the most
important compounds described in this article.
12. It was found at an early stage by Stevens1b that a low temperature and a
catalytic amount of H2SO4 were crucial for the high yield formation of m-tert-
butoxytoluene from m-cresol and isobutene. However, no yield or information
was given about the reaction conditions of other listed phenols therein (ex.
preparation of tert-butoxybenzene) which were supposedly prepared under
the same reaction conditions.
References and notes
13. O-tert-Butylation of o-benzoyloxyphenol was carried out using isobutene/TfOH
(cat.) in CH2Cl2 at ꢁ50 °C. For this, see: (a) Beech, C. L.; Coope, J. F.; Fairley, G.;
Gilbert, P. S.; Main, B. G.; Plé, K. J. Org. Chem. 2001, 66, 2240–2245; (b)
Bredikhin, A. A.; Bredikhina, Z. A.; Zakharychev, D. V.; Konoshenko, L. V.
Tetrahedron: Asymmetry 2007, 18, 1964–1970.
14. For tert-butyl aryl ethers synthesis using the tert-butyl-transfer reagent DMF
di-tert-butyl acetal, see: Mohacsi, E. Synth. Commun. 1983, 13, 827–832.
15. Preparation of 12 was carried out by reaction of t-BuCl/NaH with 2,3-
dihydroxynaphthalene. For this, see Ref. 4b.
1. (a) Greene, T. W.; Wutz, P. G. M. Protective Groups in Organic Chemistry, 3rd ed.;
John Wiley & Sons: New York, 1999; For a significant early achievement
concerning m-cresol addition to isobutene in the presence of H2SO4, see: (b)
Stevens, D. R. J. Org. Chem. 1955, 20, 1232–1236.
2. For O-tert-butylation of phenols, see: with t-BuBr/ZnO: (a) Ravindranath, B.;
Srinivas, P. Tetrahedron 1984, 40, 1623–1628. with t-BuBr/pyridine:; (b)
Masada, H.; Oishi, Y. Chem. Lett. 1978, 1, 57–58. with isobutene/TfOH (cat.),
see:; (c) Holcombe, J. L.; Livinghouse, T. J. Org. Chem. 1986, 51, 111–113. with t-
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A. S.; McClure, L. D. Tetrahedron Lett. 1997, 38, 7345–7348. with t-BuCl/Zn, see:;
(f) Bandgar, B. P.; Kasture, S. P. J. Chem. Res., Synop. 2000, 252–253. with Boc2O/
Sc(OTf)3, see:; (g) Bartoli, G.; Bosco, M.; Carlone, A.; Dalpozzo, R.; Locatelli, M.;
Melchiorre, P.; Sambri, L. J. Org. Chem. 2006, 71, 9580–9588; (h) Bartoli, G.;
Bosco, M.; Locatelli, M.; Marcantoni, E.; Melchiorre, P.; Sambri, L. Org. Lett.
2005, 7, 427–430.
3. For the preparation of tert-butoxybenzene from PhMgBr/PhCO2Ot-Bu, see: (a)
Org. Syn. Coll. 1973, 5, 924; 1965, 45, 89; (b) Lawesson, S.-O.; Yang, N. C. J. Am.
Chem. Soc. 1959, 81, 4230–4233; from PhBr/t-BuOK, see: (c) Org. Syn. Coll. 1973,
5, 926; 1961, 41, 91; (d) Cram, D. J.; Rickborn, B.; Knox, G. R. J. Am. Chem. Soc.
1960, 82, 6412–6413.
4. For 3,4-di-t-BuO-PhCO2H use (no preparation or characterization), see: (a) Rho,
H. S.; Baek, H. S.; Lee, J. A.; Park, C. M.; Kim, D. H.; Chang, S.; Lee, S.
WO2006118380, 2006; Chem. Abstr. 2006, 145, 489011; for attempts to prepare
2,3-di-tert-butoxynaphthalene from 2,3-naphthalenediol, see: (b) Font, J.;
Messeguer, A.; Serratosa, F.; Sola, P.; Vilarrasa, L. An. Quim. 1976, 72, 247–
253; for O-alkylation of phenols with Me2CClCBCH, see: (c) Kuemmerle, J.;
Jiang, S.; Tseng, B.; Kasibhatla, S.; Drewe, J.; Cai, S. X. Bioorg. Med. Chem. 2008,
16, 4233–4241.
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V. J. Org. Chem. 1967, 32, 2358–2360.
17. Crystallographic data (excluding structure factors) for the structure
(compound 22) in this Letter has been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC 898217.
Copies of the data can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223336033; e-mail: deposit@c
cdc.cam.ac.uk).
18. Surprisingly the ring-opening of oxazaPB with the bulky 2,3-di-tert-
butoxyphenyllithium was not accompanied with a competitive attack on the
ephedrine moiety as observed with other bulky organolithiums. For such
study, see: Stephan, M. M. S.; Šterk, D.; Modec, B.; Mohar, B. J. Org. Chem. 2007,
72, 8010–8018.
19. For the ring-opening of oxazaPB with polymethoxy-substituted
ˇ ˇ
phenyllithiums, see: (a) Zupancic, B.; Mohar, B.; Stephan, M. Adv. Synth.
ˇ ˇ
Catal. 2008, 350, 2024–2032; (b) Zupancic, B.; Mohar, B.; Stephan, M.
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20. (a) Jugé, S.; Stephan, M.; Laffitte, J. A.; Genêt, J. P. Tetrahedron Lett. 1990, 31,
6357–6360; (b) Stephan, M.; Modec, B.; Mohar, B. Tetrahedron Lett. 2011, 52,
1086–1089; (c) Grabulosa, A. P-Stereogenic Ligands in Enantioselective Catalysis,
1st ed.; RSC publishing: Cambridge, UK, 2011.
Please cite this article in press as: Rast, S.; et al. Tetrahedron Lett. (2012), http://dx.doi.org/10.1016/j.tetlet.2012.10.010